Validate TCP message string lengths

[kevinherron]

Motivation

• Fix a remotely-triggerable denial-of-service in OPC UA TCP Hello/Error decoding where attacker-controlled string lengths could cause large/unbounded heap allocations and negative-size allocations.
• Restore protocol and encoding limits (endpoint URL max bytes and message-size limits) and ensure frame-bounds checking happens before any allocation.

Description

• HelloMessage: validate constructor using UTF-8 encoded byte length, encode endpoint URL using UTF-8 byte length, and in decode reject negative lengths, enforce MAX_ENDPOINT_URL_LENGTH, and verify the declared length fits in buffer.readableBytes() before allocating and reading bytes.
• ErrorMessage: encode reason using UTF-8 byte length, make decode throw UaException, reject negative lengths, enforce a message-size limit (EncodingLimits.DEFAULT_MAX_MESSAGE_SIZE), and verify remaining frame bytes before allocation.
• Tests: add new ErrorMessageTest and extend HelloMessageTest with cases for negative lengths, truncated frames, and oversized lengths to cover the regression.
• Misc: adjust imports and signatures (throws UaException) where needed to propagate decoding errors.

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Codex Task